This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Mm-cpn is an Archaeal type II chaperonin of ~1 MDa. Its structural organization is simpler than TRIC, the mammalian type II chaperonin, because, while also a 16mer, it possesses only a single subunit isoform. Group II chaperonins are essential mediators of cellular protein folding in eukaryotes and archaea. These oligomeric protein machines, ~1MDa, consist of two back-to-back rings encompassing a central cavity that accommodates polypeptide substrates. Chaperonin-mediated protein folding is critically dependent on the closure of a built-in lid, which is triggered by ATP hydrolysis. The structural rearrangements and molecular events leading to lid closure are still unknown. We propose to determine its structures in different chemical states at near atomic resolution.